iMessage

iMessages are texts, photos, or videos that you send to iOS devices and Macs over Wi-Fi or cellular-data networks. These messages are always encrypted and appear in blue text bubbles. To turn iMessage on or off, go to Settings > Messages.

SMS/MMS

If you aren’t using iMessage, you can use SMS/MMS. These messages are texts and photos that you send to other cell phones or iOS devices. SMS/MMS messages aren't encrypted and appear in green text bubbles on your device.